a) Field of Invention
The present invention relates to, a process for dehydrating a material comprising a mineral oil, aromatic or aliphatic solvents, or the like using calcium carbide. More particularly, the invention relates to the preparation of samples of mineral oil or the like containing a known water concentration.
b) Description of Prior Art
The electrical properties of insulating oils, such as mineral oils, are largely affected by the presence of traces of water, which has led electrical companies to establish maximum admissible water concentrations for insulating oils. In view of this requirement, a monitoring of the water content of insulating oils is carried out so as to detect the cases which require a water removal treatment of the insulating oil. For example, the specification which is presently set by the Applicant for power transformer oils is  less than 10 ppm (w/v) water.
To achieve this, the determination of the water content is carried out by the Karl Fischer""s coulometric titration method. Although this method has a lot of merit, since it is reasonably accurate and can be advantageously employed in several fields, it has some drawbacks for this specific application. For example, when the determination is made with an oil which has been exposed to severe electric and thermal effects, the presence of degradation products with unsaturated carbon-carbon bonds which react with iodine through halogen addition causes errors in Karl Fischer""s method. In view of the problems associated with the Karl Fischei""s method, there is a need for substantially dehydrated oil samples and moisture-in-oil standards to verify the performance of the method.
Another problem with present technology is that the currently used apparatuses for detecting the water content of mineral oils are calibrated from concentrated solutions of water in methanol which could affect the analytical performance. The apparatuses should be normally calibrated in the range of the concentrations experienced with the field samples using standards prepared in the same matrix. For the present application, the oil is the matrix, whereas dissolved water, is the analyte to be determined.
There is therefore a need to obtain an accurate and precise determination of the water content of mineral oils and the like using an apparatus which has been calibrated to less than 1 ppm water with the appropriate standards.
L. Szepes et aL in xe2x80x9cA New Analytical Method for the Determination of the Water Content of Transformer Oilsxe2x80x9d (Transactions on Electrical Insulation, Vol. EI-17 No. 4, August 1982) describe a method based on the mass spectrometric determination of HD formed by reacting water with LiAID4. This analytical procedure which implies the removal of water via the above reaction does not refer to the use of calibration standards. Furthermore, it can only dry the oil down to 5 ppm.
On the other hand, U.S. Pat. No. 4,308,106 discloses that it is known to add inter alia calcium carbide in the reflux of glycerin to dehydrate same by the process of U.S. Pat. No. 1,459,699. However, no indication is given that a dehydration to less than 1ppm is possiblebythat method. Also there is no teaching relating to reducing the amount of water in transformer oil below the ppm level. This patent finds application mainly in the production of ethyl alcohol, but none to the treatment of oil.
U.S. Pat. No. 4,747,960 discloses the removal of water from a dielectric oil using a water absorbent polymer, a carboxymethyl cellulose or sodium poly-2-propenoate. However, there is no indication that water removal is possible down to sub-ppm quantities. The method deals with an absorption phenomenon and does not involve chemical reaction.
Another method of demoisturizing is disclosed in U.S. Pat. No. 4,437,082, which uses a desiccant resin. The latter presumably dries a dielectric liquid to 20 ppm or less although no showing is given to support demoistuzing at less than 20 ppm. The only dessicant material disclosed is drying resin HCR-W2. Furthermore, the patent mainly aims at decontaminating a dielectric fluid rather than essentially demoisturizing to very low levels.
U.S. Pat. No. 4,124,834 teaches the removal of a contaminant from an insulating liquid using an adsorbent material. The invention is mainly aimed at removing PCBs, and has nothing to do with the present invention.
U.S. Pat. No. 1,847,968 discloses treating alcohol with quicklime to remove water. There is no teaching relating to reducing the amount of water in transformer oil below the ppm level.
It will therefore be seen that with respect to producing mineral oil or the like which is free of water to a range below one part per million, the prior art is of no help.
It is therefore an object of the present invention to provide a method which enables to produce a sample of a material comprising a mineral oil, aromatic or aliphatic solvent, or the like which is demoisturized to the extent that it can be used to calibrate an apparatus adapted to determine the content of water in the material.
According to the invention there is provided a process for dehydrating a material comprising a mineral oil, aliphatic or aromatic solvent or the like, which comprises contacting the material with particulate calcium carbide, under conditions effective to form a liquid phase consisting of dehydrated material substantially free of water and a solid phase comprising calcium hydroxide and unreacted calcium carbide, and separating the liquid phase from said solid phase to give a substantially dehydrated material.
Although the invention may be used with a variety of liquids, it is preferably carried out with transformer oils.
According to the invention, it is possible to calibrate an apparatus for detecting the presence of water in a material such as mineral oil or an aromatic or aliphatic solvent, with the substantially dehydrated material obtained by the process of the invention.
According to a preferred embodiment calcium carbide which has been previously crushed is preferred and the process is normally carried out under anhydrous atmosphere, for example, in a glove box.
According to another preferred embodiment, transformer oil and crushed calcium carbide are introduced in centrifugal bottles, for example, bottles made of polyethylene which are closed using polyethylene screw caps. The bottles are preferably shaken mechanically (Glas-col bench-top shaker, Terre Haute, Ind., USA, equipped with centrige-tube holder) for at least about 60 minutes, and placed in an ultra-centrifuge (IEC Centra-MP4, Needham Heights, Mass., USA) to separate said liquid phase from said solid phase. The ultra-centrifuge may operate at a speed of about 13000 rpm for about 5 minutes.
According to yet another embodiment, the bottles are thereafter reintroduced in the glove box and the liquid phase comprising dehydrated oil is then decanted in additional polyethylene bottles. These additional bottles containing dehydrated oil are preferably subjected to at least two additional ultra-centrifugations to separate any remaining solid phase therefrom, and the dehydrated oil is transferred in glass bottles or syringes.
According to yet another preferred embodiment, known amounts of water are introduced into the dehydrated oil present in the glass bottles or glass syringes to produce an oil containing a known concentration of water. Each polyethylene bottle normally contains an excess of calcium carbide, such as 1 g of CaC2 per 4 mL of oil.
Samples containing different known amounts of water are required to check the accuracy and precision of apparatuses such as the Karl Fischer device or a device of the type HS/GC (A New Analytical Method for the Determination of Moisture in Transformer Oil Samples, J. Jalbert S. Charbonneau and R. Gilbert, Sixty-Third Annual International Conference of Doble Clients, Mar. 25-29, 1996, Boston, Mass., U.S.A.). To achieve this, it is necessary to have samples of substantially completely dehydrated oil and other samples containing known amounts of water both of which are not available on the market, and which makes it possible to calibrate and verify the analytical performance of measuring instruments.
In order to test the exactness of the process according to the invention in producing substantially completely dehydrated oils, 40 samples, each containing about 40 mL of Volt Esso 35 oil which have been dehydrated by the process of the invention, were prepared. An initial amount of 20 ppm of water was determined in this insulating oil before application of the process.